About 58 recombinant-inbred strains of mice have been developed from crosses between (FSxZB), (B6xCA), and (B6xC3) progenitor strains. A preliminary survey of their capacity to make an antibody response to alpha1,3-dextran and Type III pneumococcal polysaccharide (SSS-III) shows a wide spectrum of responsiveness. These strains should be useful in definitive studies on genes controlling responsiveness to these and perhaps other antigens.